1. Field of the Invention
The present invention relates to a magnetoresistive element and a method of manufacturing the same and to a thin-film magnetic head, a head gimbal assembly, a head arm assembly and a magnetic disk drive each of which incorporates the magnetoresistive element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write (recording) head having an induction-type electromagnetic transducer for writing and a read (reproducing) head having a magnetoresistive (MR) element for reading are stacked on a substrate.
MR elements include giant magnetoresistive (GMR) elements utilizing a giant magnetoresistive effect, and tunnel magnetoresistive (TMR) elements utilizing a tunnel magnetoresistive effect.
It is required that the characteristics of a read head include high sensitivity and high output capability. GMR heads incorporating spin-valve GMR elements have been mass-produced as read heads that satisfy such requirements. Recently, developments have been made for read heads using TMR elements to adapt to further improvements in areal recording density.
Typically, a spin-valve GMR element incorporates: a nonmagnetic conductive layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the surfaces of the nonmagnetic conductive layer; a pinned layer disposed adjacent to the other of the surfaces of the nonmagnetic conductive layer; and an antiferromagnetic layer disposed adjacent to one of the surfaces of the pinned layer farther from the nonmagnetic conductive layer. The free layer is a ferromagnetic layer in which the direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer in which the direction of magnetization is fixed. The antiferromagnetic layer is a layer that fixes the direction of magnetization in the pinned layer by means of exchange coupling with the pinned layer.
Conventional GMR heads have a structure in which a current used for detecting magnetic signals (that is hereinafter called a sense current) is fed in the direction parallel to a plane of each layer making up the GMR element. Such a structure is called a current-in-plane (CIP) structure. On the other hand, developments have been made for another type of GMR heads each having a structure in which a sense current is fed in the direction intersecting a plane of each layer making up the GMR element, such as the direction perpendicular to a plane of each layer making up the GMR element. Such a structure is called a current-perpendicular-to-plane (CPP) structure. A GMR element used for read heads having the CPP structure is hereinafter called a CPP-GMR element. A GMR element used for read heads having the CIP structure is hereinafter called a CIP-GMR element.
A read head incorporating the TMR element mentioned above has the CPP structure, too. Typically, a TMR element incorporates: a tunnel barrier layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the surfaces of the tunnel barrier layer; a pinned layer disposed adjacent to the other of the surfaces of the tunnel barrier layer; and an antiferromagnetic layer disposed adjacent to one of the surfaces of the pinned layer farther from the tunnel barrier layer. The tunnel barrier layer is a nonmagnetic insulating layer that allows electrons to pass therethrough while maintaining the spin by means of the tunnel effect. The free layer, the pinned layer and the antiferromagnetic layer of the TMR element are the same as those of a spin-valve GMR element.
For a conventional CPP-GMR element, a CoFe alloy and an NiFe alloy have been mostly used as the material of the pinned layer and the free layer. In such a conventional CPP-GMR element, with regard to the configuration of layers capable of achieving a practical read gap length, the magnetoresistance change ratio (hereinafter called an MR ratio), which is a ratio of magnetoresistance change with respect to the resistance, is not more than approximately four percent and therefore is not sufficient in practice.
It is assumed that the reason why the MR ratio of the above-mentioned conventional CPP-GMR element is low is that the spin polarization of the CoFe alloy or the NiFe alloy used as the material of the pinned layer and the free layer is small.
To increase the MR ratio, it has been proposed recently to employ CPP-GMR elements in which a half metal whose spin polarization is nearly 1 is used as the material of the pinned layer and/or the free layer. JP 2003-218428A, JP 2004-221526A and JP 2005-228998A disclose CPP-GMR elements in which a Heusler alloy that is a type of half metal is used as the material of the pinned layer and/or the free layer.
For TMR elements, it is also expected that a high MR ratio will be achieved by employing a half metal as the material of the pinned layer and/or the free layer. JP 2004-221526A and JP 2005-228998A disclose TMR elements in which a Heusler alloy is used as the material of the pinned layer and/or the free layer.
JP 2003-218428A discloses a structure in which magnetic layers made of a magnetic material that is any of a CoFe alloy, a CoFeNi alloy, an NiFe alloy, and Co are respectively disposed on the top and bottom of a Heusler alloy layer in a pinned layer, and magnetic layers made of a magnetic material that is any of a CoFe alloy, a CoFeNi alloy, and Co are respectively disposed on the top and bottom of a Heusler alloy layer in a free layer.
Heusler alloy will now be briefly described. Heusler alloy is a term generally used for ordered alloys having a chemical composition of XYZ or X2YZ. An ordered alloy having a chemical composition of XYZ is called a half Heusler alloy. An ordered alloy having a chemical composition of X2YZ is called a full Heusler alloy. Here, X is an element selected from the group consisting of the transition metals of the Fe family, the Co family, the Ni family and the Cu family of the periodic table, and the noble metals. Y is at least one element selected from the group consisting of Fe and the transition metals of the Ti family, the V family, the Cr family and the Mn family of the periodic table. Z is at least one element selected from the group consisting of the typical elements of the periods from the third to fifth periods inclusive of the periodic table.
One type of Heusler alloys is a CoMnSi alloy. According to the stoichiometric composition of the CoMnSi alloy as a full Heusler alloy, Co, Mn and Si are in the proportion of 2:1:1. It was expected that the MR ratio of a CPP-GMR element would be greatly increased by employing a CoMnSi alloy layer having such a proportion of the elements as the pinned layer and/or the free layer of the CPP-GMR element.
Then, a CPP-GMR element was actually fabricated using CoMnSi alloy layers in which Co, Mn and Si were in the proportion of 2:1:1 as the pinned layer and the free layer. The alloy layers were respectively formed on magnetic layers as disclosed in JP 2003-218428A. The MR ratio of the CPP-GMR element thus fabricated was not more than approximately 5 percent, which was contrary to the expectation.